This invention relates to a closed-cell rigid polyurethane foam having a remarkably low density, a low thermal conductivity and well balanced various properties obtained by using only water as a blowing agent, a process for producing such a polyurethane foam, and use thereof as heat insulator, for example, in refrigerators.
Rigid polyurethane foams are usually obtained by reacting a polyol component and an isocyanate component in the presence of a blowing agent, a reaction catalyst, and a foam stabilizer. Heretofore, as the blowing agent, there have been used fluoro carbon type blowing agents having low boiling points such as trichloromonofluoromethane (CCl.sub.3 F), etc. Buf CCl.sub.3 F is one of hardly decomposable chlorofluorocarbons (CFC) which are chlorofluoro substituted hydrocarbons. It has recently been found that when such a hardly decomposable CFC is released into the atmosphere, it acts to destroy the ozone layer in the stratosphere or cause an earth surface temperature rise due to a so-called hothouse effect. Thus, the use of such a CFC becomes a serious global environmental pollution problem in recent years. In the future, the production and consumption of the hardly decomposable CFC's are to be reduced stepwise
Thus, how to reduce their use becomes a worldwide problem.
On the other hand, it has long been known in the art that water is usable as one component of blowing agent. For example, Japanese Patent Unexamined Publication No. 53-9797 proposes the use of a fluorochlorinated hydrocarbon and water (0.2 to 3 parts by weight per 100 parts by weight of polyol component) as a blowing agent. But the use of water causes a serious problem in that its use even in a small amount caused deterioration of heat insulating properties or formation of gas boils in the product, resulting in unbalance of the properties required for a rigid urethane foam. Japanese Patent Unexamined Publication Nos. 59-84913 and 62-81414 disclose the co-use of water and CFC as a blowing agent. But in order to maintain good properties required, the amount of water is limited, for example, to less than 1% by weight in the blowing agent.
Thus, the use of water alone as the blowing agent has not been realized in industrial production of rigid polyurethane foams.